50 Ohms: The Forgotten Impedance

Posted by on 30/08/2012

If you play with coax, short for coaxial cable, you probably know this it is available in a number of different impedances. The most common is 75 ohm, like video cable or antenna cable, but in fact our products range from 32 ohms up to 124 ohms.pb299-cable-300px

Why all these different numbers? It’s not an accident of course, and there is a reason for each one. Today, we’re going to take a quick look at 50 ohm coax cable.

Belden makes hundreds of 50 ohm cables, including a whole line of ultra-low loss versions (Belden 7805 to Belden 7977). The two largest versions (Belden 7976 and 7977) are shown in the photograph below. They are HUGE. The 7977 has a diameter of .600″ six-tenths of an inch! This is the largest coax cable that we make.

But first of all, why 50, or any other number? The answer can be shown in the graph below. This was produced by two researchers, Lloyd Espenscheid and Herman Affel, working for Bell Labs in 1929. Read more »

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Splitting Digital Video

Posted by on 24/05/2012

Back in the old days of analog audio, splitting a signal was no big deal. Just use a ‘Y’ adaptor. Or if you’re punching down wires, just punch another pair on top of the first. Of course, each signal will be 3 dB lower than the original, but that’s not a worry. That’s because the wavelength of audio is miles long (quarter-wavelength at 20 kHz is more than 2 miles!).

While adding wires might change the impedance, it didn’t matter. You couldn’t go far enough for it to matter. But when it came to video, it was a different story. The signal is a lot higher frequency, so the wavelength is shorter. While you might get by with a BNC ‘T’, this would cause a mismatch on the two splits and could result in some reflected signals.   Read more »

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Impedance and Return Loss

Posted by on 04/05/2012

When you deal with high frequencies, above around 100 MHz, you have entered the zone of the “transmission line.” It has to do with the wavelength of the signal, and that is a discussion we’ll have in future blogs. But what it means is that the impedance of the cable is now important and you have to match the impedance of the source and destination devices.

This also means that everything in-between must match the impedance chosen and, by “everything,” I mean cable, connectors, patch panels, patch cords, adaptors, bulkheads, feedthroughs – erything! It also means that any variation in impedance can affect the signal on the line. This is true for every transmission line, whether we’re talking about a 50kW RF signal going up to an antenna or an HD video signal going between boxes. Of course, in these two examples the impedance is different (50 ohms for that high-power line and 75 ohms for that video cable). Read more »

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